Does it shut down when you press the button w/o girder running?
I've messed up a serial port circuit before which caused my computer to shut down, but it was instant - no software problems appeared. So I would *guess* that the command you're using in Girder is causing problems. Try changing the command to something simple, or not binding it to anything at all.
Oh - that a hella-big circuit board, dude! I wired up almost an identical thing (same parts) in a 9-pin d-sub.